GENERATION OF OPTICAL SIGNALS WITH RF COMPONENTS

Description

[0001] The present invention relates to a method of generating signals having optical carriers. It finds particular application in the generation of radio frequency (RF) modulations to be carried over optical media.

[0002] The principle of modulating an optical beam by an information content is well established, and various methods are known of achieving this. Some arrangements involve controlling the light source (typically a laser) by varying its input bias voltage. Other arrangements use optical devices in the path of the generated beam (typically on an optical fibre) to interrupt the beam. A known optical device for this purpose is the Mach-Zehnder interferometer. The principle of this device is to split the optical beam into two paths, one or both of which passes through a medium whose refractive index varies as a function of the electrical potential applied to it. By applying an electrical signal to one or both paths, the difference between the path lengths of the two beam paths can be varied, such that the re-combined beams interfere constructively or destructively depending on the electrical fields applied. The intensity of the recombined beam therefore varies in response to the varying electrical input signal.

[0003] It is known for the modulation carried by an optical signal to include a carrier frequency in the radio frequency (RF) range. This principle, known as 'Radio by Fibre,' allows a radio signal, including its RF carrier, to be generated at one location and transmitted over the air from another, remote, location. The signal is typically carried by an optical fibre between these locations. This allows the equipment at the point from which the signal is to be transmitted over the air to be kept very simple. In its simplest form it need consist only of a detector to convert the optical input into an electrical signal, and an antenna for transmitting the electrical signal over the air. This is particularly useful in situations where an antenna has to be located at a point difficult of access, such as a hilltop, because the complex equipment required to generate the radio frequency carrier in particular the local oscillator can be located at another more accessible location. Moreover, it is possible to achieve economies in a branched network, in which one signal is transmitted to several antenna sites, because only one local oscillator is required to generate the carrier to be transmitted by all the antennas.

[0004] The detectors for these arrangements are typically photodectors. These produce an electrical output which varies with the intensity of incident light. This electrical output therefore corresponds to the modulation, but without the optical carrier frequency.

[0005] Known optical systems suffer from a number of practical limitations, in particular in the accurate transmission of high radio frequencies (of the order of a few tens of GHz). As frequencies approach the millimetre waveband (tens of gigahertz) it becomes increasingly difficult to achieve direct modulation of the laser by applying a signal to the input bias voltage, because of inherent physical limitations of the laser devices themselves. Similar constraints apply to modulation devices such as the Mach-Zehnder interferometers discussed above, as the high frequencies necessitate very small dimensions which impose constraints which reduce their efficiency. Velocity matching between the electrical and optical signals also becomes harder to achieve and maintain.

[0006] There is an additional problem with the application of signals by means of a Mach-Zehnder optical modulator. As explained above, the principle of these modulators is that as the voltage applied to the electrical input of a Mach-Zehnder interferometer is increased, the difference in optical path length increases. This results in the two optical paths passing in and out of phase, so that the amount of light passing through the interferometer varies as a periodic function of the applied voltage, and not as a linear function. This non-linear response to the input means that only constant-amplitude signals would be accurately reproduced.

[0007] A proposal by O'Reilly and Lane (Electronics letters, Vol 28, No 25, page 2309) addresses the first of these problems. In this proposal, an electrical signal having a frequency ω in the RF band is applied to the control input of an optical modulator. This modulator is biassed such that the optical output generated is modulated by a signal dominated by two side bands centred on the optical carrier frequency of the original optical signal fed into the modulator and each spaced from the optical carrier frequency by the frequency of the electrical signal ω. These side bands produce sum and difference beats at an optical receiver such as a photodector. The 'sum' beat is at twice the optical carrier frequency. The 'difference' beat is at frequency 2ω which is in the RF band. A photoelectric receiver would not be sensitive to the optical-frequency 'sum' beat, but would generate an electrical signal at the 'difference' frequency. This proposal therefore produces an output signal carried by the optical beam which is at twice the frequency of the electrical signal applied to the control input.

[0008] O'Reilly and Lane further propose applying an information-containing modulation to this output signal by separating the two side bands using optical filters, modulating one of them with the information content using a second optical modulator, and re-combining them, to generate an output having an optical carrier, modulated by a second, radio frequency, carrier 2ω, and further modulated by the information content. However, this further optical modulation suffers from the non-linearity discussed above, and requires the use of optical filters and a second optical modulator which result in optical losses.

[0009] According to a first aspect of the invention there is provided a method of generating a modulated optical signal having a first RF component including a first RF carrier frequency and an information component, the method comprising the following steps: -

i) Generating a first optical signal having a second RF component including a second RF carrier frequency different from said first RF carrier frequency;

ii) Generating a control signal having a third RF component including a third RF carrier frequency different from said first RF carrier frequency;

   said second or said third RF component including the information content;

iii) applying the first optical signal to an optical modulator and

iv) applying the control signal to the optical modulator to modulate the first optical signal so as to produce an output optical signal modulated by said first RF carrier frequency and said information component, said first carrier frequency being said second carrier frequency upconverted by said third carrier frequency or by an integer multiple of said third carrier frequency.

[0010] The invention has a number of advantages over the prior art. By applying RF components to both inputs of the modulator, an upconversion is achieved. This allows a higher frequency to be generated at the output of the modulator than is applied to either of the inputs. The use of lower RF frequencies in the optical input allows a simpler laser to be used to generate the desired RF signal. The use of lower RF frequencies at the control input of the modulator similarly allows greater flexibility and simplicity in the design of the modulator, which is constrained by the input frequency and not the output frequency.

[0011] Either the first optical signal or the control signal may include the information component. The invention also extends to a method of encryption comprising the steps of the first aspect wherein both the input optical signal and the control signal include an information component, one of which is a predetermined encryption code, and to a method of de-encrypting a signal generated in this way, comprising applying to the output signal a further modulation complementary to that of the predetermined encryption code. This provides a simple way of upconverting and encrypting a signal in one operation.

[0012] In a preferred embodiment the control signal is an electrical signal, the modulator being of the type where the optical output of the modulator has a non-linear response to the electrical control input. A Mach-Zehnder interferometer is an example of such a modulator. Using a modulator of this type the first RF carrier frequency may simply be a sum of the second and third RF carrier frequencies (i. e the third RF carrier frequency upconverts the second RF carrier frequency to the first RF carrier frequency). However in this type of modulator the amplitude of the control signal may be selected such that the second RF component is upconverted by a frequency which is a desired integer multiple of the third RF component. This allows even larger conversion factors to be created between the control frequency and the output frequency, with consequent lower frequency control signals, further mitigating the design constraints on the modulator.

[0013] The modulation carried by the first optical signal is preferably generated by controlling the bias voltage of a laser.

[0014] The invention also extends to an optical signal generated according to the method of the invention, and a radio or electrical signal generated by detecting such an optical signal.

[0015] According to a second aspect the invention comprises apparatus for generating an optical signal having a first RF component which includes a first RF carrier frequency and an information component, comprising;

i) an optical modulator having an optical input, an optical output, and a control input;

ii) means for supplying to the optical input a modulated optical signal having a second RF component including a second RF carrier frequency different from said first RF carrier frequency;

iii) means for supplying to the control input a control signal having a third RF component including a third RF carrier frequency different from said first RF carrier frequency,

iv) means for applying a modulation comprising said information component to said optical signal or said control input;

   the arrangement being such that there is produced at the optical output an optical signal modulated by said first RF carrier frequency and said information component, said first carrier frequency being said second carrier frequency upconverted by said third carrier frequency or by an integer multiple of said third carrier frequency.

[0016] In a preferred embodiment the control input is an electrical input and the optical modulator is a Mach-Zehnder interferometer. The means for generating the control signal may be arranged to generate a signal of sufficient amplitude that the second RF component is upconverted by a frequency which is an integer multiple of the third RF component.

[0017] The means for supplying the optical signal is preferably a laser. Preferably means for controlling the bias voltage of the laser are provided for generating the second RF component.

[0018] The input optical signal therefore carries an initial RF modulation, so that the output optical signal comprises an RF modulation which is the initial RF modulation upconverted by the control RF frequency. This allows the use of control frequencies lower than the desired output RF carrier frequency. Moreover, if a modulator having a non-linear transfer function is used, such as a Mach-Zehnder interferometer (whose transfer function is periodic) even larger upconversion factors can be used because by selection of the amplitude of the control signal appropriately, the output can be dominated by a harmonic of the control signal. To understand why this occurs, consider a control signal having a amplitude V_2π causing the difference in path lengths in the optical paths to vary between zero and one wavelength λ. (See Figure 2). On each cycle of the control signal the path length difference will thus vary from zero to λ and back again. There will thus be two points of constructive interference (at zero and V_2π) and two points of destructive interference (at V_π) for each cycle of the control signal, so that the signal applied by the modulator to the optical signal is in this case twice the control signal frequency. By selecting other amplitudes for the control signal different integer multiplication factors can be introduced. In the simple example above the amplitude of the control signal is chosen to vary the path length by a whole number of wavelengths. Varying it over smaller amplitudes can also generate signals having dominant harmonics which may be used in the same way.

[0019] The invention also extends to an encryption device comprising the elements of the second aspect of the invention wherein means are provided for applying a modulation comprising an encryption code to the optical input or the control input.

[0020] Information content may be applied to either input signal. In modulators such as Mach-Zehnder interferometers having the non-linearity referred to above, large amplitude modulations of the control input would not be accurately reproduced in the output optical signal. However amplitude modulations applied to the optical input do have a linear response. The Mach-Zehnder interferometer used as the modulator of the preferred arrangement also has a linear response to phase and frequency modulations applied to either the control or the optical input. Combinations, such as phase-amplitude modulation (e. g quadrature amplitude modulation: QAM) are also possible in the optical input.

[0021] According to a third aspect of the invention, there is provided a method of generating an output optical signal having an RF modulation comprising applying an input optical signal to an optical input of a modulator having a non-linear transfer function, applying a control signal having a control RF frequency to a control input of the modulator, the amplitude of the control signal being such that the output optical signal is modulated by an RF frequency which is an integer multiple of the control RF frequency. The modulator is preferably a Mach-Zehnder interferometer. This aspect of the invention allows high RF frequency modulations to be applied to the optical signal although a lower frequency is applied to the control input. This has the advantages discussed above in mitigating design constraints on the optical modulator.

[0022] The invention will now be further described by way of example only with reference to the accompanying drawings in which;

Figure 1 is a diagrammatic representation of an arrangement for performing the method of the invention;

Figure 2 is a diagram showing the variation of transmissivity of a Mach-Zehnder modulator to changes in applied voltage; and

Figures 3 to 6 are diagrams showing the way in which the frequency multiplication factor changes with the amplitude of the voltage applied to a Mach-Zehnder modulator.

[0023] Referring to Figure 1, there is shown an arrangement including a laser 1 having an electrical power input 2, and an optical output 3. A source 4a of an RF electrical signal is connected to the electrical input 2 through amplifier 5. Connected to the optical output 3 of laser 1 is a Mach-Zehnder optical modulator 6. The electrical input 7 of the modulator 6 is fed another RF signal from source 4b through a power amplifier 9. The output 10 of the modulator 6 is connected, through optical fibre 11 to a photodetector 12 which converts the optical signal to an electrical signal. The photodetector is coupled through another amplifier 13 to a radio antenna 14 which converts the electrical signal to a radio signal. The RF electrical signals from either of source 4a or source 4b may include a modulation carrying the required information content. Signal source 4a may generate analogue or digital modulation outputs which may themselves be modulated onto RF carrier frequencies. Signal source 4a may generate a multichannel output using any suitable modulation scheme such as frequency modulation, amplitude modulation, or phase modulation. Because of the non-linearity of the modulator, source 4b can only supply one channel at a time. This channel may be frequency or phase modulated.

[0024] Several ways in which this arrangement may be used will now be described.

[0025] In a first method the signal source 4a generates an electrical FM carrier in the gigahertz range suited to the response time of the laser 1. The FM electrical carrier signal carries a modulation in the megahertz band, and provides the input to the laser 1, the optical signal generated by the laser 1 varying directly in response to the electrical input signal to provide a modulated FM optical signal at output 3.

[0026] In order to up-convert the optical signal modulated at an intermediate frequency (IF) to a higher frequency the optical signal is mixed in modulator 6 with a local oscillator frequency LO from source 4b. The output of modulator 6 is thus a signal comprising an optical carrier, modulated by a high frequency RF signal being the RF carrier frequency (IF) from source 4a plus the local oscillator frequency of source 4b, itself modulated by the information content. Upconversion therefore takes place within the optical system, and this has a number of advantages over performing it in the electrical systems upstream or downstream of the optical system. In a second arrangement, the information bearing modulation may be applied through signal source 4b. This can be phase or frequency modulated and has an RF carrier frequency.

[0027] By supplying the signal from source 4b at a sufficiently large amplitude the RF frequency may be multiplied in the modulator 6 in a manner to be described below, allowing the output of the modulator to be at a higher frequency than the electrical input, thereby avoiding the problems associated with optical modulators when driven at such high electrical frequencies.

[0028] Other arrangements can be devised which are within the scope of this invention. For example, information-bearing signals may be applied by both sources 4a and 4b, that at source 4b being a predetermined code. The two signals will become scrambled in output 10. A remote user, knowing the code signal applied at 4b may receive the scrambled signal over the air from antenna 14, and re-combine the output signal by a signal complementary to that from source 4b, to recover the signal from input 4a.

[0029] The use of the modulator as a multiplier of the local oscillator frequency, in addition to its function as a mixer of the local oscillator and optical modulation signals, will now be described.

[0030] In figure 2 the horizontal axis shows the voltage applied to the modulator and the vertical axis shows the transmissivity of the modulator. It will be seen that the response of the modulator is highly non-linear with respect to variations in electrical input. It is therefore most suited for provision of a constant amplitude modulation. More complex modulations will be distorted by the non-linear response. In arrangements in which the modulated input 4 is fed to the electrical input 7 of the optical modulator 6, this non-linear response limits use of the system to single channel applications.

[0031] In the arrangement according to the present invention, the non-linear response of the modulator can be used to generate harmonics of the local oscillator frequency, thus allowing higher upconversion factors in the modulator.

[0032] The output i(t) of the photodector 12 can be expressed as a sum of its Fourier components I_p:

[0033] Assume a Mach-Zehnder modulator with a characteristic symmetrical about V=0 (Fig.2), with V_π being the voltage excursion required for the transmissivity of the Mach-Zehnder interferometer to go from full transmission (constructive interference) to full extinction (destructive interference).

[0034] Applying a sinusoidal voltage (V_a sinωt + V_b) to such an interferometer where:

V_a = amplitude of applied voltage

V_b = d.c bias of applied voltage

gives an output having harmonics whose Fourier amplitudes are then given by:

[0035] By selecting the bias voltage to be ½(V_π) we can generate even-only harmonics. By selecting the bias voltage to be V₀ we can select odd-only harmonics. Selecting V_a = 0 reproduces the Mach-Zehnder transfer function in the zeroth harmonic as the bias voltage V_b is tuned, and zero for the higher harmonics.

[0036] The d.c amplitude is equal to |I₀|. (ie the mean light power transmitted through the modulator)

[0037] The a.c amplitude is equal to |2I_p|, p>1.

[0038] Thus we can define the modulation depth for the "p'th" harmonic as:

However the value of I₀ changes with applied a.c. modulation voltage V_a (as well as with bias voltage V_b). Therefore, maximising the modulation depth does not necessarily correspond to maximising the amplitude of a particular harmonic.

[0039] It is perhaps more convenient to choose the d. c. level when V_a = V_b = 0 as the reference, in which case I₀ (V_a = V_b = 0) = 1.

[0040] Then our modified modulation depth becomes:

[0041] Figures 3 to 6 show transfer characteristics calculated for various values of V_a and V_b. Odd harmonics are biassed at ½V_π, even ones are biassed at V=0. In these figures the input voltage is shown as a dotted line and the output as a solid line. The voltages applied (arbitrary units) are given in the Table below:-

Figure	Va	Vb	Principal Harmonic
3	1	½Vπ	Fundamental
4	59	0	2nd Order
5	112	½Vπ	3rd Order
6	877	0	10th Order

[0042] In this way, by applying different amplitudes V_a to the electrical input, modulations of different frequencies can be produced in the optical system.

[0043] As will be seen, the output wave form is not the same shape as the input. It can therefore be seen from these Figures that multiple-channel signals applied to such a modulator input would be distorted and thus difficult to extract at the receiver. However,for a single-frequency input such as a local oscillator this is not important as unwanted harmonics can be filtered out downstream.

[0044] In the simple case described with reference to Figures 3 to 6 the optical input is unmodulated, so that the optical output is modulated only by the multiplied control frequency. However, if the optical input signal already carries a modulation, the optical modulator will mix this modulation with the multiplied control frequency to provide an upconversion.

EXAMPLE

[0045] In the exemplary embodiment of Figure 1, the signal source (4a) was embodied by an Avantek VTO 9090 oscillator generating channels between 950-1750 MHz. The output from this was used to modulate a Lasertron QLXS 1300 MW laser (1), whose output was directed along a step-index single mode 9/125 µm optical fibre (3) to a BT&D IOC 2000-1300 modulator (6). The control input (7) to this modulator was supplied by a Marconi 2042 local oscillator (4b) working at 3.4 GHz and amplified by a Minicircuits ZFL/42 amplifier (9) such that the eighth harmonic of the local oscillator frequency (ie 27.2 GHz) dominated the signal response. The output of modulator 6 thus had a RF carrier frequency of 27.2 GHz + (950 to 1750 MHz) or 28.15 to 28.95 GHz, which was fed through another step-index single mode 9/125 µm optical fibre 11 to a detector (12) such as described in Wake D: "A 1550nm Millimetre - wave Photodetector with a Bandwidth Efficiency Product of 2.4 THz". (Journal of Lightwave Technology, 1992, Vol 10 pages 908-912). The output from this detector was amplified by a Celeritek CSA946892 amplifier (13) and transmitted from a standard gain 20dBi horn antenna as a microwave transmission in the 28GHz band.

[0046] While the embodiments described above have all included Mach-Zehnder interferometers, those skilled in the art will appreciate that the configuration of the interferometer is not significant; any type of interferometer which exhibits non-linear transmission characteristics such as electro-absorption modulators may be used instead. All that is required is that it should exhibit an appropriate transmission characteristic.

Claims

1. A method of generating an optical signal (10) having a first RF component including a first RF carrier frequency and an information component, the method comprising the following steps:-

i) generating a first optical signal (3) having a second RF component including a second RF carrier frequency different from said first RF carrier frequency;

ii) generating a control signal (7) having a third RF component including a third RF carrier frequency different from said first RF carrier frequency;

   said second or said third RF component including the information content;

iii) applying the first optical signal (3) to an optical modulator (6) and

iv) applying the control signal (7) to the optical modulator (6) to modulate the first optical signal so as to produce an output optical signal (10) modulated by said first RF carrier frequency and said information component, said first carrier frequency being said second carrier frequency upconverted by said third carrier frequency or by an integer multiple of said third carrier frequency.

2. A method according to claim 1, wherein the first optical signal (3) includes the information component.

3. A method according to claim 1, wherein the control signal (7) includes the information component.

4. A method according to claim 1, being a method of encryption, wherein both the input optical signal (3) and the control signal (7) include an information component, one of which is a predetermined encryption code.

5. A method according to any preceding claim, wherein the control signal (7) is an electrical signal.

6. A method according to any preceding claim, wherein the optical output (10) of the modulator (6) has a non-linear response to the control signal (7).

7. A method according to claim 6, wherein the amplitude of the control signal (7) is selected such that the second RF carrier frequency is upconverted to the first RF carrier frequency by a frequency which is an integer multiple of the third RF frequency.

8. A method according to any preceding claim, wherein the first optical signal (3) is generated by controlling the bias voltage of a laser (1).

9. A modulated optical signal (10) having a first RF component including a first RF carrier frequency and an information component, when generated by the method of any preceding claim.

10. An electrical or radio signal having a first RF carrier frequency and an information component, generated by detecting an optical signal (10) modulated by an RF component comprising a first RF carrier frequency and the information component, the optical signal (10) being generated by the method of any of claims 1 to 8.

11. A method of de-encrypting a signal (10) according to claim 9 or claim 10 when generated according to the method of claim 4 or any claim dependent thereon, comprising applying to the signal (10) a further modulation complementary to that of the predetermined encryption code.

12. Apparatus for generating an optical signal having a first RF component, which includes a first RF carrier frequency and an information component, comprising

i) an optical modulator (6) having an optical input (3), an optical output (10), and a control input (7);

ii) means (1) for supplying to the optical input (3) a modulated optical signai having a second RF component including a second RF carrier frequency different from said first RF carrier frequency;

iii) means for supplying to the control input (7) a control signal having a third RF component including a third RF carrier frequency different from said first RF carrier frequency,

iv) means (4a, 4b) for applying a modulation comprising said information component to said optical signal (3) or said control input (7);

   the arrangement being such that there is produced at the optical output (10) an optical signal modulated by said first RF carrier frequency and said information component, said first carrier frequency being said second carrier frequency upconverted by said third carrier frequency or by an integer multiple of said third carrier frequency.

13. An encryption device comprising apparatus according to claim 12, further comprising means (4a, 4b) for applying modulation comprising information components to both the optical signal and the control signal, one of the information components being a predetermined encryption code.

14. Apparatus according to claim 12 or 13 wherein the control input (7) is an electrical input.

15. Apparatus according to claim 14, wherein the optical modulator (6) is a Mach-Zehnder interferometer.

16. Apparatus according to claim 15, wherein the means (9) for supplying the control signal (7) is arranged to generate a signal such that the second RF component is upconverted by a frequency which is an integer multiple of the third RF component to generate the first RF component.

17. Apparatus according to any of claims 12 to 16, wherein the means for supplying the modulated optical signal comprises a laser (1) and means (5) for controlling the bias voltage of the laser (1) for generating the second RF component.

18. A method of generating an output optical signal (10) having an RF modulation, the method comprising applying an input optical signal to an optical input (3) of a modulator (6) having a non-linear transfer function, applying a control signal having a control RF frequency to a control input (7) of the modulator (6), the amplitude of the control signal being such that the output optical signal is modulated by an RF frequency which is an integer multiple of the control RF frequency.

19. A method according to claim 18, wherein the modulator (6) is a Mach-Zehnder interferometer.

20. A method according to claim 18 or claim 19, wherein the input optical signal (3) carries an initial RF modulation, the output optical signal (10) comprising an RF modulation which is the initial RF modulation upconverted by an integer multiple of the control RF frequency.

Ansprüche

1. Verfahren zur Erzeugung eines optischen Signals (10) mit einer ersten RF-Komponente mit einer ersten RF-Trägerfrequenz und einer Informationskomponente mit folgenden Schritten:

i) Erzeugen eines ersten optischen Signals (3) mit einer zweiten RF-Komponente mit einer zweiten RF-Trägerfrequenz, die sich von der ersten RF-Trägerfrequenz unterscheidet;

ii) Erzeugen eines Steuersignals (7) mit einer dritten RF-Komponente mit einer dritten Trägerfrequenz, die sich von der ersten RF-Trägerfrequenz unterscheidet, wobei die zweite oder die dritte RF-Komponente den Informationsgehalt aufweisen;

iii) Anlegen des ersten optischen Signals (3) an einen optischen Modulator (6) und

iv) Anlegen des Steuersignals (7) zur Modulation des ersten optischen Signals an den optischen Modulator (6), damit ein optisches Ausgangssignal (10) erzeugt wird, das durch die erste RF-Trägerfrequenz und die Informationskomponente moduliert ist, wobei die erste Trägerfrequenz der durch die dritte Trägerfrequenz oder ein ganzzahliges Vielfaches der dritten Trägerfrequenz nach oben gewandelten zweiten Trägerfrequenz entspricht.

2. Verfahren nach Anspruch 1, wobei das erste optische Signal (3) die Informationskomponente aufweist.

3. Verfahren nach Anspruch 1, wobei das Steuersignal (7) die Informationskomponente aufweist.

4. Verfahren nach Anspruch 1, das ein Verschlüsselungsverfahren ist, wobei jeweils das optische Eingabesignal (3) und das Steuersignal (7) eine Informationskomponente aufweisen, wobei eine von beiden ein vorbestimmter Verschlüsselungscode ist.

5. Verfahren nach einem der vorherigen Ansprüche, wobei das Steuersignal (7) ein elektrisches Signal ist.

6. Verfahren nach einem der vorherigen Ansprüche, wobei die optische Ausgabe (10) des Modulators (6) eine nichtlineare Antwort auf das Steuersignal (7) aufweist.

7. Verfahren nach Anspruch 6, wobei die Amplitude des Steuersignals (7) so gewählt wird, daß die zweite RF-Trägerfrequenz durch eine Frequenz, die ein ganzzahliges Vielfaches der dritten RF-Frequenz ist, zur ersten RF-Trägerfrequenz nach oben umgewandelt wird.

8. Verfahren nach einem der vorherigen Ansprüche, wobei das erste optische Signal (3) erzeugt wird, indem die Vorspannung eines Lasers (1) gesteuert wird.

9. Moduliertes optisches Signal (10) mit einer ersten RF-Komponente mit einer ersten RF-Trägerfrequenz und einer Informationskomponente, das nach dem Verfahren einer der vorherigen Ansprüche erzeugt wird.

10. Elektrisches Signal oder Radiosignal mit einer ersten RF-Trägerfrequenz und einer Informationskomponente, das durch die Detektion eines optischen Signals (10) erzeugt wird, das durch eine RF-Komponente mit einer ersten RF-Trägerfrequenz und der Informationskomponente moduliert wurde, wobei das optische Signal (10) durch das in einem der Ansprüche 1 bis 8 beanspruchte Verfahren erzeugt wird.

11. Verfahren zur Entschlüsselung eines Signals (10) nach Anspruch 9 oder 10, das gemäß dem Verfahren von Anspruch 4 oder jedem davon abhängigen Anspruch erzeugt wurde, wobei eine weitere Modulation auf das Signal (10) aufgeprägt wird, die komplementär zu dem vorbestimmten Verschlüsselungscode ist.

12. Vorrichtung zur Erzeugung eines optischen Signals mit einer ersten RF-Komponente, die eine erste RF-Trägerfrequenz und eine Informationskomponente aufweist, mit

i) einem optischen Modulator (6) mit einer optischen Eingabe (3), einer optischen Ausgabe (10) und einer Steuereingabe (7);

ii) einer Einrichtung (1), die ein moduliertes optisches Signal mit einer zweiten RF-Komponente mit einer zweiten RF-Trägerfrequenz, die sich von der ersten RF-Trägerfrequenz unterscheidet, an die optische Eingabe (3) anlegt;

iii) einer Einrichtung, die ein Steuersignal mit einer dritten RF-Komponente mit einer dritten RF-Trägerfrequenz, die sich von der ersten RF-Trägerfrequenz unterscheidet, an die Steuereingabe (7) anlegt;

iv) Einrichtungen (4a, 4b), die dem optischen Signal (3) oder der Steuereingabe (7) eine Modulation mit der Informationskomponente aufprägen;

wobei die Anordnung derart ist, daß bei der optischen Ausgabe (10) ein optisches Signal erzeugt wird, das durch die erste RF-Trägerfrequenz und die Informationskomponente moduliert wurde, wobei die erste Trägerfrequenz der zweiten Trägerfrequenz entspricht, die durch die dritte Trägerfrequenz oder ein ganzzahliges Vielfaches der dritten Trägerfrequenz nach oben umgewandelt wurde.

13. Verschlüsselungsvorrichtung mit einer Vorrichtung nach Anspruch 12, die weiterhin Einrichtungen (4a, 4b) aufweist, die eine Modulation mit Informationskomponenten jeweils auf das optische Signal und das Steuersignal aufprägen, wobei eine der Informationskomponenten ein vorbestimmter Verschlüsselungscode ist.

14. Verfahren nach Anspruch 12 oder 13, wobei die Steuereingabe (7) eine elektrische Eingabe ist.

15. Vorrichtung nach Anspruch 14, wobei der optische Modulator (6) ein Mach-Zehnder-Interferometer ist.

16. Vorrichtung nach Anspruch 15, wobei die Einrichtung (9) zum Anlegen des Steuersignals (7) so zur Erzeugung eines Signals angeordnet ist, daß die zweite RF-Komponente zur Erzeugung der ersten RF-Komponente durch eine Frequenz nach oben umgewandelt wird, die ein ganzzahliges Vielfaches der dritten RF-Komponente ist.

17. Vorrichtung nach einem der Ansprüche 12 bis 16, wobei die Einrichtung, die das modulierte optische Signal anlegt, einen Laser (1) und eine Einrichtung (5) aufweist, die die Sperrspannung des Lasers (1) zur Erzeugung der zweiten RF-Komponente steuert.

18. Verfahren zur Erzeugung eines optischen Ausgabesignals (10) mit einer RF-Modulation, wobei ein optisches Eingabesignal an eine optische Eingabe (3) eines Modulators (6) mit einer nichtlinearen Übertragungsfunktion angelegt wird und ein Steuersignal mit einer RF-Steuerfrequenz an eine Steuereingabe (7) des Modulators (6) angelegt wird, wobei die Amplitude des Steuersignals so gewählt wird, daß das optische Ausgabesignal durch eine RF-Frequenz moduliert wird, die ein ganzzahliges Vielfaches der RF-Steuerfrequenz ist.

19. Verfahren nach Anspruch 18, wobei der Modulator (6) ein Mach-Zehnder-Interferometer ist.

20. Verfahren nach einem der Ansprüche 18 oder 19, wobei das optische Eingabesignal (3) eine erste RF-Modulation trägt, wobei das optische Ausgabesignal (10) eine RF-Modulation aufweist, die der ersten RF-Modulation entspricht, die durch ein ganzzahliges Vielfaches der RF-Steuerfrequenz nach oben umgewandelt wurde.

Revendications

1. Procédé de génération d'un signal optique (10) comportant une première composante à fréquence radio comprenant une première fréquence de porteuse à fréquence radio et une composante d'informations, le procédé comprenant les étapes suivantes :

i) générer un premier signal optique (3) comportant une seconde composante à fréquence radio comprenant une seconde fréquence de porteuse à fréquence radio différente de ladite première fréquence de porteuse à fréquence radio,

ii) générer un signal de commande (7) comportant une troisième composante à fréquence radio comprenant une troisième fréquence de porteuse à fréquence radio différente de ladite première fréquence de porteuse à fréquence radio,

   ladite seconde composante ou ladite troisième composante à fréquence radio comprenant le contenu d'informations,

iii) appliquer le premier signal optique (3) à un modulateur optique (6) et

iv) appliquer le signal de commande (7) au modulateur optique (6) afin de moduler le premier signal optique de façon à produire un signal optique en sortie (10) modulé par ladite première fréquence de porteuse à fréquence radio et ladite composante d'informations, ladite première fréquence de porteuse étant ladite seconde fréquence de porteuse transposée en fréquence par ladite troisième fréquence de porteuse ou par un multiple entier de ladite troisième fréquence de porteuse.

2. Procédé selon la revendication 1, dans lequel le premier signal optique (3) comprend la composante d'informations.

3. Procédé selon la revendication 1, dans lequel le signal de commande (7) comprend la composante d'informations.

4. Procédé selon la revendication 1, qui est un procédé de cryptage, dans lequel à la fois le signal optique en entrée (3) et le signal de commande (7) comprennent une composante d'informations, dont l'une est un code de cryptage prédéterminé.

5. Procédé l'une quelconque des revendications précédentes, dans lequel le signal de commande (7) est un signal électrique.

6. Procédé selon l'une quelconque des revendications précédentes, dans lequel la sortie optique (10) du modulateur (6) présente une réponse non linéaire au signal de commande (7).

7. Procédé selon la revendication 6, dans lequel l'amplitude du signal de commande (7) est choisie de sorte que la seconde fréquence de porteuse à fréquence radio soit transposée en fréquence en la première fréquence de porteuse à fréquence radio par une fréquence qui est un multiple entier de la troisième fréquence à fréquence radio.

8. Procédé selon l'une quelconque des revendications précédentes, dans lequel le premier signal optique (3) est généré en commandant la tension de polarisation d'un laser (1).

9. Signal optique modulé (10) comportant une première composante à fréquence radio comprenant une première fréquence de porteuse à fréquence radio et une composante d'informations, lorsqu'il est généré par le procédé selon l'une quelconque des revendications précédentes.

10. Signal électrique ou à fréquence radio comportant une première fréquence de porteuse à fréquence radio et une composante d'informations, généré par la détection d'un signal optique (10) modulé par une composante à fréquence radio comprenant une première fréquence de porteuse à fréquence radio et la composante d'informations, le signal optique (10) étant généré par le procédé selon l'une quelconque des revendications 1 à 8.

11. Procédé de décryptage d'un signal (10) conforme à la revendication 9 ou la revendication 10 lorsqu'il est généré conformément au procédé de la revendication 4 ou de l'une quelconque des revendications dépendantes de celle-ci, comprenant l'application au signal (10) d'une modulation supplémentaire complémentaire de celle du code de cryptage prédéterminé.

12. Dispositif destiné à générer un signal optique comportant une première composante à fréquence radio, qui comprend une première fréquence de porteuse à fréquence radio et une composante d'informations, comprenant

i) un modulateur optique (6) présentant une entrée optique (3), une sortie optique (10), et une entrée de commande (7),

ii) un moyen (1) destiné à appliquer à l'entrée optique (3) un signal optique modulé comportant une seconde composante à fréquence radio comprenant une seconde fréquence de porteuse à fréquence radio différente de ladite première fréquence de porteuse à fréquence radio,

iii) un moyen destiné à fournir à l'entrée de commande (7) un signal de commande comportant une troisième composante à fréquence radio comprenant une troisième fréquence de porteuse à fréquence radio différente de ladite première fréquence de porteuse à fréquence radio,

iv) un moyen (4a, 4b) destiné à appliquer une modulation comprenant ladite composante d'informations audit signal optique (3) ou à ladite entrée de commande (7),

l'agencement étant tel qu'il se produit au niveau de la sortie optique (10) un signal optique modulé par ladite première fréquence de porteuse à fréquence radio et ladite composante d'informations, ladite première fréquence de porteuse étant ladite seconde fréquence de porteuse transposée en fréquence par ladite troisième fréquence de porteuse ou par un multiple entier de ladite troisième fréquence de porteuse.

13. Dispositif de cryptage comprenant un dispositif conforme à la revendication 12, comprenant en outre un moyen (4a, 4b) destiné à appliquer une modulation comprenant des composantes d'informations à la fois au signal optique et au signal de commande, l'une des composantes d'informations étant un code de cryptage prédéterminé.

14. Dispositif selon la revendication 12 ou 13 dans lequel l'entrée de commande (7) est une entrée électrique.

15. Dispositif selon la revendication 14, dans lequel le modulateur optique (6) est un interféromètre de Mach-Zehnder.

16. Dispositif selon la revendication 15, dans lequel le moyen (9) destiné à appliquer le signal de commande (7) est agencé pour générer un signal tel que la seconde composante à fréquence radio soit transposée en fréquence par une fréquence qui est un multiple entier de la troisième composante à fréquence radio afin de générer la première composante à fréquence radio.

17. Dispositif selon l'une quelconque des revendications 12 à 16, dans lequel le moyen destiné à appliquer le signal optique modulé comprend un laser (1) et un moyen (5) destiné à commander la tension de polarisation du laser (1) afin de générer la seconde composante à fréquence radio.

18. Procédé de génération d'un signal optique en sortie (10) présentant une modulation à fréquence radio, le procédé comprenant l'application d'un signal optique en entrée à une entrée optique (3) d'un modulateur (6) comportant une fonction de transfert non linéaire, l'application d'un signal de commande comportant une fréquence de commande à fréquence radio à une entrée de commande (7) du modulateur (6), l'amplitude du signal de commande étant telle que le signal optique en sortie est modulé par une fréquence à fréquence radio qui est un multiple entier de la fréquence de commande à fréquence radio.

19. Procédé selon la revendication 18, dans lequel le modulateur (6) est un interféromètre de Mach-Zehnder.

20. Procédé selon la revendication 18 ou la revendication 19, dans lequel le signal optique en entrée (3) porte une modulation initiale à fréquence radio, le signal optique en sortie (10) comprenant une modulation à fréquence radio qui est la modulation initiale à fréquence radio transposée en fréquence par un multiple entier de la fréquence de commande à fréquence radio.

Drawing